1. Field of the Invention
The present invention relates to a voltage supplying device, and a semiconductor device, an electro-optical device and an electronic instrument using the voltage supplying device.
2. Description of Related Art
In recent years, there are devices requiring a highly accurate voltage supply, for example, a liquid crystal display.
In an active matrix type of liquid crystal display or a simple matrix type of liquid crystal display, the number of grayscales (or colors) of a liquid crystal panel is highly improved, and a voltage to be applied is developed to be more precise.
In order to increase the number of grayscales of a liquid crystal panel, a thin film transistor (TFT) liquid crystal device which is an active matrix type of liquid crystal display uses three colors of RGB (red, green and blue) which includes data signals constituted of 6-bit data (64 grayscales, ca. 260,000 colors) or 8-bit data (256 grayscales, ca. 16,770,000 colors), for example.
With the increase in the number of grayscales, a technique of setting a voltage level more precisely is required.
According to the characteristics in relationship between the applied voltage and the panel transmittance of a liquid crystal panel, the rate of change in panel transmittance with respect to the applied voltage is large where the transmittance is in the middle level around 50%, and is decreased when the panel transmittance approaches 100% or 0%. Therefore, in the region where the panel transmittance is in the middle level, a slight deviation in the applied voltage has a great influence on a gray level. In order to suppress the change in panel transmittance, a voltage to be applied to liquid crystal is required to be supplied more precisely.
The maximum permissible level in dispersion of a voltage to be applied to liquid crystal is xc2x15 mV for 64 grayscales and xc2x11 to xc2x12 mV for 256 grayscales, for example, and more precise voltage is required to be applied to liquid crystal when the number of grayscales is increased. Although the dispersion in the threshold voltage VTH of an ordinary IC chip is allowed to range from several tens mV to several hundreds mV, a liquid crystal display with the increased number of grayscales has a severer maximum permissible level. It can be predicted that further increase in the number of grayscales in future requires more precise setting method for a voltage to be applied to liquid crystal.
Consequently, there are conventionally various method of generating grayscale voltages in a driving circuit of a liquid crystal panel, such as a voltage selecting method, a time sharing method, or a digital-analog conversion method.
FIG. 4 shows a conventional voltage supplying device of the method using a digital-analogue conversion device (hereinafter referred to as a DAC method).
A voltage follower circuit 72, into which an output from a DAC 70 is entered, functions as an impedance converter, and in the case of a voltage follower circuit 72 in an ideal state, a voltage of a node 201 entered into a non-inverse input terminal becomes equal to a voltage of a node 202 entered into an inverse input terminal. However, conventionally in the voltage follower circuit 72 not compensated by an offset canceling circuit, an offset is formed between the input and the output due to dispersion in performance of respective transistors, so as to form a difference in voltage between the node 201 and the node 202.
FIG. 4 shows a voltage supplying device for solving the problem. The output from the DAC 70 is supplied to the non-inverse input terminal 201 of the voltage follower circuit 72, and the output of the voltage follower circuit 72 is returned to the inverse input terminal 202. In the course of the circuit connecting the output line and the non-inverse input terminal 201, a switching element Q10, a capacitance C10 and a switching element Q12 are connected in series. On the negative feedback line connected to the inverse input terminal 202, only a switching element Q1 is present. The switching element Q10 is connected in parallel to the capacitance C10 and the switching element Q11.
In a first period, the switching element Q11 is of f, and the switching element Q10 and the switching element Q12 are on, whereby an offset voltage between the input and the output of the voltage follower circuit 72 is charged in the capacitance C10. In a second period, the switching element Q11 is on, and the switching element Q10 and the switching element Q12 are off, whereby a charge of offset canceling charged in the capacitance C10 is superposed and returned to the inverse input terminal 202 of the voltage follower circuit 72.
According to the foregoing manner, the offset is cancelled out by applying a reverse voltage corresponding to the offset in such a manner that the capacitance C10 for offset canceling is provided on the circuit connecting the output line and the non-inverse input terminal 201 of the voltage follower circuit 72.
In the data driver of the conventional DAC method shown in FIG. 4, the capacitance C10 as the offset canceling circuit is necessarily housed in the chip. However, it requires a large area since the capacitance C10 having a sufficiently larger capacity than the input capacity of the voltage follower circuit 72. When the offset canceling capacity is too small, it is regarded as a noise in the input capacity of the voltage follower circuit 72, and thus the noise is superposed on the output voltage.
Furthermore, in order to charge the offset voltage in the offset canceling capacitance C10, a period of time of from 3 to 5 xcexcs is generally required.
In the active matrix type of liquid crystal display of these kinds, the horizontal scanning period (select period) is necessarily set at a short period when high definition display is conducted by increasing the number of pixels in one line. For example, the select period becomes as short as from 8 to 12 xcexcm in high definition display of SXGA.
In this case, it becomes difficult to assure a period of time for offset canceling when the period for charging the capacitance C10 for offset canceling occupies the select period.
The invention has been developed taking the problems into consideration, and an objective thereof is to provide a voltage supplying device that can promptly and precisely provide a required charging voltage without an offset canceling circuit, and a semiconductor device, an electro-optical device and an electronic instrument using the same.
According to a first aspect of the present invention, there is provided a voltage supplying device which supplies a voltage to a load capacitance to finish charging the load capacitance with a predetermined voltage within a predetermined charging period. The voltage supplying device comprises: a voltage supplying source; an impedance conversion circuit which performs impedance conversion for a voltage from the voltage supplying source and outputs the converted voltage; a first switching element connected between the impedance conversion circuit and the load capacitance; a bypass line for bypassing the impedance conversion circuit and the first switching element and supplying a voltage from the voltage supplying source to the load capacitance; and a second switching element provided on the bypass line. The first switching element is turned on and the second switching element is turned off in the first period of the charging period, and the first switching element is turned off and the second switching element is turned on in the second period of the charging period which follows after the first period.
According to this aspect of the present invention, the output voltage from the impedance conversion circuit is supplied to the load capacitance through the first switching element in the first period of the charging period. If an offset is present between the input and output voltages of the impedance conversion circuit, the load capacitance will not be charged with the predetermined voltage even when the output voltage from the impedance conversion circuit is continuously supplied to the load capacitance.
Thus, the route for voltage supplying is switched to the bypass route in the second period of the charging time, whereby the voltage from the voltage supplying source is directly supplied to the load capacitance without using the impedance conversion circuit. Accordingly, the load capacitance is supplied with a voltage compensating the shortage caused by the offset and can be charged with the predetermined voltage. The charge amount per unit period of time supplied from the voltage supplying source to the load capacitance is decreased since the impedance conversion is not performed. However, if the load capacitance has been charged with a sufficient voltage by the output voltage from the impedance conversion circuit, the load capacitance can be charged to the predetermined voltage within the charging period.
Furthermore, because a capacitance for offset canceling used in the conventional technique is not necessary, a period of time for charging the capacitance for offset canceling with an offset voltage is not necessary.
There may be a period in which both the first and second switching elements are turned off. This makes it possible to prevent positive feedback of the voltage from the voltage supplying source through the bypass line to the impedance conversion circuit.
The voltage supplying device may further comprise a third switching element connected on a power source line which supplies a power source voltage to the impedance conversion circuit. The third switching element is turned off, synchronized with an off operation of the first switching element. This makes it possible to stop the power supply when the output from the impedance conversion circuit is unnecessary, so as to reduce power consumption.
The impedance conversion circuit may be formed of a voltage follower circuit. When an input voltage having a magnitude near a power source potential VDD or a ground potential VEE is input to the voltage follower circuit, such voltage follower circuit has a property in which an output voltage is saturated and shows no linear characteristics in response to an input voltage. In this case, a voltage from the voltage supplying source is supplied to the load capacitance through the bypass line by turning off the first switching element and turning on the second switching element in a saturated region of an output voltage of the voltage follower circuit. This makes it possible to supply a linear output voltage by directly outputting a voltage from the voltage supplying source in the saturated region in which an output voltage is saturated with respect to a lower or higher input voltage in the voltage follower circuit.
In order to generate a linear output voltage when using the above-described voltage follower circuit, the voltage supplying device may further comprise a comparator for comparing an output voltage from the voltage supplying source with an output voltage from the voltage follower circuit. The first and second switching elements can be controlled according to a result of comparison by the comparator, enabling to output a voltage from the voltage supplying source instead of the saturated voltage.
According to a second aspect of the present invention, there is provided a semiconductor device comprising the above-described voltage supplying device. In the semiconductor device, a capacitance for offset canceling is unnecessary, so that the chip size can be reduced by the area of the capacitance or other elements can be integrated on the area of the capacitance to increase the degree of integration.
According to a third aspect of the present invention, there is provided an electro-optical device comprising a display section using an electro-optical element and a semiconductor device which is provided with the above-described voltage supplying device, wherein the semiconductor device is used as a driver IC for driving a signal line of the display section. A precise driving voltage can be supplied to the electro-optical element by supplying a voltage from the voltage supplying source through a signal line of the display section to the electro-optical element.
In this case, the electro-optical element may be driven based on grayscale voltages from the voltage supplying device. The voltage supplying source can be formed of a digital-analog converter which converts a digital grayscale signal to an analogue voltage. The first period of the charging period may be finished after the load capacitance is charged with a voltage which has a magnitude within a range corresponding to half of the least signification bit with respect to a desired grayscale voltage value to be supplied to the electro-optical element and which has a magnitude of 90% or more of the desired grayscale voltage value. When a sufficient voltage is supplied to the electro-optical element in the first period of the charging period, the applied voltage to the electro-optical element can reach the desired grayscale voltage even when the voltage from the DA converter is directly supplied to the load capacitance in the second period of the charging period, and furthermore, the gray level in the electro-optical element can be prevented from being differentiated.
According to a fourth aspect of the present invention, there is provided an electronic instrument comprising the above described electro-optical device. Image quality can be improved by using the electro-optical device as a display of the electronic instrument.